1. Technical Field
The present invention relates to an electrophotographic photoreceptor used in a field of copying machines or printers, and a device using the electrophotographic photoreceptor.
2. Description of Related Art
Because an electrophotographic photoreceptor (hereinafter, simply referred to as a photoreceptor) is greatly advantageous in that the selection range of materials is wide, the environmental suitability is excellent, the production cost is low, etc., as compared with an inorganic photoreceptor such as a selenium photoreceptor or an amorphous silicon photoreceptor, recently, the main stream of the electrophotographic photoreceptors is shifted to organic photoreceptors in place of inorganic photoreceptors.
On the other hand, in recent electrophotographic image forming methods, a digital image forming method using LED or laser as an image exposure light source is rapidly spread for a hard copy printer of personal computer or an usual copying machine due to ease of image processing as well as ease of expansion to a complex copying machine, and a technique for forming an electrophotographic image having high image quality by a digital system has been developed. For example, there is disclosed a technique in which image exposure is carried out using a laser beam having a small spot area, the density of a dotted latent image is increased to form a latent image with high precision, and the latent image is developed by a toner having a small particle diameter to thereby form an electrophotographic image having high image quality (JP Tokukai 2001-255685A).
Furthermore, down sizing and speed-up of recent electrophotographic apparatuses such as digital copying machines and printers have been progressed, and both of high sensitivity corresponding to a high speed tendency and a long life by improvement in abrasion resistance have been demanded as photoreceptor properties.
In order to meet the above-described requirements for high image quality, down sizing and high speed tendency, organic photoreceptors must have adaptability to reversal development most suitable for the development of digital latent images as well as charging property and electrophotographic properties such as high sensitivity and low dark decay.
In order to meet the above-described requirements for high image quality, down sizing and high speed tendency, time response properties of sensitivity of the photoreceptor must be elevated. In order to meet these requirements, organic photoreceptors are designed to have a layer construction where the photosensitive layer is functionally separated into a charge generation layer and a charge transportation layer, and thereby, pursuing high sensitivity and a high-speed property of the charge generation layer and the charge transportation layer.
However, when these charge generation materials or charge transportation materials having high sensitivity and a high-speed property are used, there arises a problem that stability in charging property or sensitivity is easily deteriorated. Specifically, there arises a problem that charging potential is liable to decrease or residual potential is liable to increase under a high temperature and high humidity environment or a low temperature and low humidity environment.
More specifically, when the stability in the charging potential or the sensitivity is deteriorated, a difference between the potential at the unexposed portion (VH) and the potential at the exposed portion (VL) on the photoreceptor is reduced to result in decrease in image density and at the same time, a potential difference between the potential at the unexposed portion (VH) and the direct current bias potential (VDC) applied between the photoreceptor and the developing sleeve is also reduced to result in easy occurrence of image defects such as black spots, in the reversal developing process.
In order to solve the above-described problems in the stability of charging potential and the image defects such as black spots, a technique of using an interlayer in the organic photoreceptor is developed. For example, there is known an organic photoreceptor having a construction where an interlayer comprising a resin and a titanium oxide particle dispersed in the resin is provided between an electroconductive substrate and a photosensitive layer. Further, a technique of using an interlayer comprising a surface-treated titanium oxide is also known. For example, there is proposed a organic photoreceptor having an interlayer using titanium oxide surface-treated by iron oxide or tungsten oxide (for example, JP Tokukaihei 4-303846A), titanium oxide surface-treated by an amino group-containing coupling agent (for example, JP Tokukaihei-9-96916A), titanium oxide surface-treated by an organosilicon compound (for example, JP Tokukaihei-9-258469A), titanium oxide surface-treated by a methylhydrogenpolysiloxane (for example, JP Tokukaihei-8-328283A), or branch-shaped titanium oxide surface-treated by a metal oxide or an organic compound (for example, JP Tokukaihei-11-344826A).
However, even when these foregoing techniques are applied, formation of black spots cannot be sufficiently prevented under serious environments such as a high temperature and high humidity. Moreover, there arise problems such that a sufficient image density cannot be obtained as a result of increase in the residual potential and the potential at the exposed portion accompanied with repetition of the use.
Further, it has been proposed to control more accurately the crystalline structure of titanium oxide and to improve the above-described problems such as formation of black spots or increase in the residual potential and the potential at the exposed portion accompanied with repetition of the use. For example, an interlayer comprising an anatase-type titanium oxide pigment (hereinafter referred to also as an anatase-type titanium oxide or an anatase-type titanium oxide particle) has been proposed (for example, JP Tokukaihei-11-327188A). The anatase-type titanium oxide has a low volume resistivity as compared with rutile-type titaniumu oxide and therefore, the interlayer can be formed in a large film thickness. By covering irregularity on the electroconductive support with the thickness, the injection of charges from the electroconductive support is easily blocked, on the contrary, an increase of fog in the reversal development is liable to be caused as a result of increase in dark decay of charging potential. In this Patent Document, sufficient solutions to such opposed problems cannot be found yet.
In addition, a method of forming an interlayer by dispersing the above-described titanium oxide particle, etc. in a polyamide resin is widely known. However, a copolymer polyamide resin or a methoxymethylated polyamide resin, which is usually used as a polyamide resin in this case and composed of a chemical structure having a small number of carbon chains between amide bonds, such as 6-nylon is high in a water absorption coefficient. Therefore, the interlayer formed by using such a polyamide has a tendency to be increased in the environmental dependency, as a result, formation of black spots, etc. easily occurs due to increase in the residual potential accompanied with repetition of the use or easy change in charging property under high temperature and high humidity conditions.
A copolymer polyamide resin comprising a constituent element having a large number of carbon chains between amide bonds, such as 12-nylon resin is low in a water absorption coefficient. Therefore, it is expected that the resin is a useful material for producing a photoreceptor having low environmental dependency. However, such a polyamide is insoluble in a usual organic solvent and therefore, is not suitable for the production of photoreceptors. There is an example of using a polyamide improved in solubility by methoxymethylation (for example, JP Tokukaihei-5-72787A and JP Tokukaihei-6-186767A), however, it is difficult to sufficiently reduce the formation of black spots, etc. because methoxymethylation remarkably increases the water absorption coefficient.